This invention relates in general to an improved mandrel for transporting hollow cylinders and, more specifically, to a mandrel and process for using the mandrel for coating hollow cylinders.
Although this invention is especially useful for the fabrication of electrostatographic imaging members, it is not limited to such application. Electrostatographic imaging members are will known in the art and include electrophotographic imaging members and electrographic imaging members.
Electrophotographic imaging member may be in the shape of a hollow drum or cylinder and are coated with at least one active electrophotographic imaging layer. The active electrophotographic imaging layer may comprise a single photoconductive layer or comprise a plurality of active electrophotographic layers such as a charge generating layer and a charge transport layer. These drum shaped electrophotographic imaging members embodiments are well known in the art.
Electrostatographic drums are conventionally coated by immersing hollow cylinders into a liquid coating solution, withdrawing the cylinders coated with the coating solution and drying the coating on the cylinders. Generally, the coating applied to the cylinder is confined to the exterior surface of the cylinder to conserve coating material, to avoid heat build-up, and to provide an internal contact surface for electrical contact for electrical grounding or biasing. To prevent any significant deposition of coating material onto the interior of the cylinder during immersion of the substrate into the coating bath, the cylinder axis is maintained in a vertical position or attitude and air within the hollow cylinder is trapped in at least the lower section of the interior of the cylinder by various known techniques. Trapping of the air in the cylinder while the cylinder axis is maintained in a vertical attitude minimizes wasteful deposition of coating material within the interior of the cylinder. One technique for trapping air within the cylinder is to insert the lower end of a mandrel into the upper open end of a cylinder, the mandrel having an expandable component positioned at or adjacent to its lower end which can be expanded to firmly contact and grip the interior of the cylinder to form a seal which traps air in the section of the cylinder below the seal during immersion of the cylinder in a coating liquid. One such technique is described in U.S. Pat. No. 4,680,246, the entire disclosure thereof being incorporated herein by reference. In another embodiment, the expandable mandrel component has a shape similar to that of a disk or thick washer. The outside diameter of this expandable disk shaped member, in its normal unstressed state, is slightly less than the interior diameter of the cylinder that is transported by the mandrel. The axis of this disk is coaxial with the main mandrel body. The expandable disk is expanded by applying compressive pressure on at least a segment of the upper and lower surfaces of the disk. The applied compression pressure causes the length of the circumference of the circular outermost edge of the disk to increase sufficiently whereby the outer periphery of the disk firmly contacts and grips the interior surface of the cylinder so that the mandrel can support and carry the cylinder from one location to another and also function as an air seal to trap air within the interior of the cylinder in the section below the cylinder when the cylinder is immersed in a liquid coating bath. The compressive pressure may be applied to the disk by applying a pulling or tension force on a tension shaft extending from a presser means through the center of the disk and through the mandrel body whereby the disk is squeezed by the presser means against the adjacent lower end of the mandrel body. The use of a mandrel that supports a hollow cylinder by gripping only the interior of the cylinder facilitates coating of the exterior surface of the cylinder without any mechanical object contacting either the exterior surface of the cylinder or the coating deposited on the exterior surface. The mandrel is supported at its upper end by any suitable conventional conveyor means. The conveyor means can comprise means to raise and lower the mandrel and/or the entire conveyor means may be raised and lowered by any suitable and conventional means such as an elevator means.
Generally, in fabricating electrophotographic imaging members, the cylindrical substrate is transported to a cleaning station where it is cleaned. After cleaning, the substrate is transported to a coating station where a coating is applied. After application of the coating, the coated substrate is transported to a drying station maintained at an elevated temperature to dry off the solvent used during application of the coating. Upon completion of the drying steps, the photoreceptor is subsequently transported one additional time to another coating station and drying station. One conventional technique for coating cylindrical substrates is by dipping the cylinder into a bath of liquid coating material. Upon removal of the substrate from the coating bath, it is transported to a heating oven which can be, for example, a forced air oven. The common technique for coating the coated cylinder into the drying zone is to place the freshly coated cylinders onto a platen. The platen is normally transported on a conveyor through the drying zone.
During the dip coating operation, the cylindrical substrate is supported from above by a mandrel arm which extends into the interior of the cylindrical substrate and expands and grips the inner walls of cylindrical substrate. The upper hand of the mandrel is secured to a conventional conveyor system which raises and lowers the mandrel for purposes of dipping the cylinder into a coating bath and thereafter withdrawing the cylinder. The various steps of loading cylinders onto a mandrel, unloading them onto a platen, loading them back up onto another mandrel for another coating step and unloading onto another platen for a drying step clearly increases the number separate loading and unloading steps which in turn increases the likelihood that the substrate or coating thereon be damaged during the multiple handling steps. However, when attempts are made to maintain the cylindrical substrate on the same mandrel through a coating, drying, second coating, second drying, etc. steps, difficulties have been encountered with the formation of uneven coatings and/or coatings with electrical properties which are non-uniform from one end of the coated drum to the other end. These uneven properties profiles along the length of the coated cylinder render the coated cylinder unsuitable for high quality imaging systems.
It has been found that a portion of the cylindrical substrate above the expanded gripping disk of the mandrel took longer to warm up during the drying operation than the portion of the cylindrical substrate below the gripping disk. Also, the portion of the cylindrical substrate above the gripping disk of the mandrel tended to cool more slowly after removal from the hot drying zone. The mandrel formed a heat sink that absorbed heat that retarded heating of the upper region of the cylinder during the heat drying step and radiated heat that retarded cooling of the upper section of a coated hollow cylinder relative to the effects occuring in the lower section of a coated hollow cylinder during heating and cooling. Also, cold air tramped in the interior upper section of the coated cylinder also retarded heating of the upper section of the hollow cylinder compared to the lower section of the coated hollow cylinder. Further, after the drying treatment, hot air tramped in the upper section of the coated hollow cylinder tended to retard cooling relative to the cooling rate of the lower section of the coated substrate. Also, heat energy conduction through the expansion disk from the interior surface of the cylindrical substrate to the mandrel also retarded heating of the cylinder during the drying operation and heat energy conduction through the expansion disk from the the mandrel to the interior surface of the cylindrical substrate retarded cooling during the cooling operation following the drying step. Further, heat energy radiated from the interior surface of the cylindrical substrate to the mandrel also retarded heating of the cylinder during the drying operation and heat energy radiation from the the mandrel to the interior surface of the cylindrical substrate retarded cooling during the cooling operation following the drying step.